Microstructure and mechanical property of 738 low alloy steel welded joint

Abstract

This paper aims to investigate the microstructure, microhardness and impact toughness of the gas-tungsten arc welding (GTAW) 738 low alloy steel joint before and after post-weld heat treatment (PWHT). The microstructure of the weld metal of the as-welded (AW) joint consists of acicular ferrite, bainite and martensite-austenite (M-A) constituents. The fraction of each phase in the weld metal of AW joint and PWHT joint was quantitatively measured by Mössbauer spectroscopy. The grain size of the weld metal obviously increases and M-A constituents decompose after PWHT. The carbides precipitate along ferrite boundaries and inside ferrite grains. The microhardness of the weld metal and heat affected zone (HAZ) obviously decreases after PWHT. PWHT promotes the grain growth and the carbide precipitation, which results in a lower impact toughness for the PWHT joint compared with the AW joint.